1. Field of the Invention
The present invention relates generally to an optical pickup apparatus that diffracts each of a first laser light beam and a second laser light beam which are different in wavelength by a diffraction grating disposed in a common optical path in an outward path which guides the first laser light beam and the second laser light beam, and that applies the diffracted light beams to an optical disc. The present invention relates more particularly to an optical pickup apparatus having a first grating that has diffracting action on the first laser light beam and a second grating that has diffracting action on the second laser light beam, both the gratings being formed as the diffraction grating.
2. Description of the Related Art
In an optical pickup apparatus that optically records and reproduces a signal to/from an optical disc such as a CD (Compact Disc) or a DVD (Digital Versatile Disc) with use of a laser light beam, an astigmatism method or a differential astigmatism method which is an application thereof are mainly employed as focusing control for causing the laser light beam applied to the disc to focus on signal layer of the disc.
On the other hand, as tracking control for causing the laser light beam applied to the disc to trace a signal track of the disc, is employed mainly any one of a 3-beam method, a push-pull method, a phase difference method, and an application thereof, in accordance with the standard and the scheme of the disc.
A practical example of an optical pickup apparatus supporting various types of CD for recording and reproducing, a DVD-ROM, and a DVD±R/RW employs, e.g., the astigmatism method for various types of CD for recording and reproducing, as the focus controlling scheme. Another practical example of the optical pickup apparatus employs, e.g., the differential astigmatism method that is an application of the astigmatism method for various types of DVD for recording and reproducing, as the focus controlling scheme. On the other hand, a practical example of the optical pickup apparatus employs, e.g., the phase difference method for a DVD-ROM and the differential push-pull method that is an application of the push-pull method for a DVD±R/RW and a CD, as the tracking controlling scheme.
When the basic form or an application form of the astigmatism method is employed for the focusing control, there is provided an astigmatism-generating optical system such as an anamorphic lens for generating astigmatism in a reflected laser light beam from the disc or a parallel plate disposed being inclined to the optical axis, and there is used a photo detector having a light receiving area that is divided into four by two dividing lines which are perpendicular to each other and each of which forms an angle of 45 degree relative to the direction of the generation of the astigmatism.
On the other hand, the differential astigmatism method for the focusing controlling scheme, and the differential push-pull method or the 3-beam method for the tracking controlling scheme, each needs three beams as the laser light beam to be applied to the disc. For this reason, in an optical pickup apparatus employing one of these methods, as already known, there is provided a diffraction grating for diffracting a laser light beam emitted form a semiconductor laser to be divided into three beams that are a 0th-order light beam and ±1st-order diffracted light beams, and there is used a photo detector having three light receiving units for respectively receiving three reflected laser light beams, that are the above three beams reflected by the disc.
In the optical pickup apparatus, incidentally, an optical path thereof is simplified, with use of a multi-laser unit that emits at least two different types of laser light beam: a laser light beam having a wavelength suitable for the recording density of a CD; and a laser light beam having a wavelength suitable for the recording density of a DVD. The multi-laser unit has a plurality of laser emitting points at which a plurality of laser light beams different in wavelength are respectively emitted by a single laser chip or by a plurality of laser chips, each of which is formed separately, and which are disposed together on a single semiconductor base.
In an optical pickup apparatus using such a multi-laser unit as above, in order to form the three beams, there is disposed a two-wavelength supporting diffraction grating provided with separate gratings so as to support different wavelengths of the first laser light beam and the second laser light beam respectively, wherein the two laser light beams are emitted by a two-wavelength supporting multi-laser, and wherein the separate gratings are formed on the outward path that guides the laser light beams to the optical disc. Because of forming different gratings respectively for the first laser light beam and the second laser light beam, the two-wavelength supporting diffraction grating is advantageous for optical design in terms of the diffraction efficiency, the diffraction angle, etc. Therefore, when the multi-laser unit is used, it is preferable to use a diffraction grating that supports a plurality of wavelengths and that is designed optimally for each of all wavelengths emitted from the multi-laser unit.
Such a conventional optical pickup apparatus is disclosed in, e.g., Japanese Patent Application Laid-Open Publication No. 2003-162831.
However, when the above diffraction grating that supports a plurality of wavelengths is used, both of the first laser light beam and the second laser light beam emitted from the multi-laser unit pass through both of the gratings of the diffraction grating. Therefore, unnecessary diffracted light beams are generated at the passing the diffraction grating. Consequently, an unnecessary diffracted light beam spot of the unnecessary diffracted light beam is projected onto the light receiving area of the photo detector, and this becomes a factor that disturbs light receiving outputs obtained respectively from sub-light receiving units corresponding to a front sub-beam and a rear sub-beam (generally, ±1st-order diffracted light beams) in the light receiving area, wherein the front sub-beam and the rear sub-beam are positioned respectively in front of and at the back of the main beam of the three beams.
With respect to an optical pickup apparatus supporting CDs and DVDs, in the case that a multi-laser unit thereof can emit a laser light beam with an infrared wavelength band of 765 nm to 805 nm which is suitable for CDs, and can also emit another laser beam with a red wavelength band of 645 nm to 675 nm which is suitable for DVDs, the use of a two-wavelength supporting diffraction grating for CDs and DVDs results in a projection of the unnecessary diffracted light beam spot onto a position on the main light receiving unit side in each of the sub-light receiving units of a light receiving are a for CDs, due to the passage of the laser light beam for CDs through the DVD-supporting grating of the diffraction grating. Thereby, an effect of disturbing the light receiving output obtained from each of the sub-light receiving units is significant. In particular, there are formed light receiving areas corresponding respectively to laser light beams emitted from the multi-laser unit in the photo detector. Therefore, when a multi-laser unit is used, a position of each of the light receiving units of each of the light receiving areas is limited corresponding to a position of each of laser emitting points of the multi-laser unit, and the effect of the unnecessary diffracted light beam spot exerted on the light receiving output is significant, wherein the light receiving output is obtained from each of the sub-light receiving units of each of the light receiving areas.